This invention relates to a regulation and intensity controller for an incandescent lamp and in particular to such a controller for use with a high intensity incandescent lamp.
All incandescent lamps have a lifetime that is sensitive to the voltage at which the lamp is operated. This is particularly true of high intensity lamps such as the type used in operating rooms and movie projectors. For example, one such high intensity lamp is estimated to operate for approximately forty hours at 21 volts.sub.RMS and for only one hour at 23 volts.sub.RMS. However, the lumen output of such a lamp is proportional to the square of the operating voltage, and therefore a decrease in voltage below the rated operating voltage may also be undesirable if a high intensity light source is desired.
The true measure of the voltage applied to a lamp, which is essentially a pure resistance device, is root mean square voltage. Accordingly, in order to assure a constant voltage applied to a lamp, it is necessary to maintain a constant V.sub.RMS across the lamp terminals. Controllers that monitor V.sub.RMS and regulate the output provided to a lamp at a rated V.sub.RMS level are commercially available. Such controllers are expensive and are typically used n conjunction with a step-down voltage transformer, which adds additional cost to the controller. An alternative is to apply a controlled DC voltage to the lamp because V.sub.DC is equal to V.sub.RMS and, thus, by controlling the level of the DC voltage the level of the V.sub.RMS is also controlled. Known DC power supplies are expensive and usually dissipate an excessive amount of power, which may, in some supplies, be equivalent to the power applied to the lamp. Importantly, the use of a constant-current DC power supply for a lamp has been observed to give rise to "filament notching" phenomena, which is the appearance of step-like or sawtooth irregularities appearing on the filament surface and which may reduce the life of the bulb to one-half, or much less, of its design-predicted value. This phenomena is believed the result of the normal decrease in filament diameter, and hence increase in impedance, which occurs during the life of a typical incandescent lamp. As the impedance increases, the voltage across the lamp, and hence the wattage applied to the lamp, will increase due to the constant-current characteristics of many DC power supplies. The increase in wattage causes an increase in the operating temperature of the lamp which tends to decrease life span.
Accordingly, it is an object of the present invention to provide an incandescent light regulation and intensity controller which provides a controlled V.sub.RMS voltage source for an incandescent lamp while avoiding the difficulties of the prior art controllers discussed above. More specifically, it is an object of the present invention to provide a controller which produces stable V.sub.RMS source for an incandescent lamp without requiring the use of expensive and bulky components, such as transformers. It is a further object to provide such a controller which causes negligible power loss through dissipation of excess power, is insensitive to a wide range to fluctuations in supply voltage, which additionally regulates lamp current, and which does not produce voltage overshoot, especially during the warm-up period of the lamp.